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          <h1 class="post-title" itemprop="name headline">js高程读书笔记 第二章 在html中使用JavaScript</h1>
        

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        <p>[TOC]</p>
<h1 id="使用script元素"><a href="#使用script元素" class="headerlink" title="使用script元素"></a>使用script元素</h1><p>6种属性可选</p>
<ul>
<li>async 表示立即下载脚本</li>
<li>charset 指定的代码的字符集</li>
<li>defer 表示脚本可以延迟到文档完全被解析和显示之后再执行。</li>
<li>src 包含要执行代码的外部文件</li>
<li>type 看成是language的替代属性。表示变写代码使用的脚本语言的内容类型(mime) 。实际上，服务器在传送JavaScript文件时使用的MIME类型通常是application/x-javascript,考虑到约定俗成和随最大限度的浏览器兼容性，依旧使用text/javascript。</li>
<li>language 已弃用</li>
</ul>
<p>在引用外部文件的时候，如果是XHTML文档中可以省略，例如</p>
<a id="more"></a>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">&lt;script type=&quot;text/javascript&quot; src=&quot;example.js&quot; /&gt;</span><br></pre></td></tr></table></figure>
<p>当然这种语法不符合HTML规范。</p>
<p>如果在script标签种包含了嵌入代码，只会下载并执行外部脚本文件。</p>
<p>只要不存在defer和async属性，浏览器都会按照script元素在页面种出现的先后顺序对它们进行依次解析。</p>
<h2 id="标签的位置"><a href="#标签的位置" class="headerlink" title="标签的位置"></a>标签的位置</h2><p>一般都把全部JavaScript引用放在<body>元素页面内容的后面</body></p>
<h2 id="延迟脚本"><a href="#延迟脚本" class="headerlink" title="延迟脚本"></a>延迟脚本</h2><p>defer属性的用途是表明脚本在执行时不会影响页面的构造，就是说脚本会被延迟到整个页面都解析完毕后再运行<br><figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br></pre></td><td class="code"><pre><span class="line">&lt;!DOCUMENT html&gt;</span><br><span class="line">&lt;html&gt;</span><br><span class="line">  &lt;head&gt;</span><br><span class="line">    &lt;script type=&quot;text/javascript&quot; defer=&quot;defer&quot;  src=&quot;example1.js&quot;&gt;&lt;/script&gt;</span><br><span class="line">  &lt;/head&gt;</span><br><span class="line">  &lt;body&gt;</span><br><span class="line">  &lt;/body&gt;</span><br><span class="line">&lt;/html&gt;</span><br></pre></td></tr></table></figure></p>
<blockquote>
<p>defer属性只适用于外部脚本文件</p>
</blockquote>
<h2 id="async属性"><a href="#async属性" class="headerlink" title="async属性"></a>async属性</h2><p>同样只适用于外部脚本文件，不同点在于标记为async的脚本并不保证按指定它们的先后顺序执行。</p>
<h2 id="在XHTML中的平稳退化"><a href="#在XHTML中的平稳退化" class="headerlink" title="在XHTML中的平稳退化"></a>在XHTML中的平稳退化</h2><p>在XHTML中，&lt;（小于）会被当做开始一个新标签来解析。有两种方法来避免类似的语法错误：</p>
<ol>
<li>用相应的HTML实体(&lt;)替代&lt;（小于）</li>
<li>CData片段来替代<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br></pre></td><td class="code"><pre><span class="line">&lt;script&gt;</span><br><span class="line">//&lt;![CDATA[</span><br><span class="line">  function compare(a, b) &#123;</span><br><span class="line">      if (a &lt; b)&#123;</span><br><span class="line">          alert(&quot;a比b小&quot;);</span><br><span class="line">      &#125; else if (a &gt; b) &#123;</span><br><span class="line">          alert(&quot;a比b大&quot;);</span><br><span class="line">      &#125; else &#123;</span><br><span class="line">          alert(&quot;a和b一样大&quot;);</span><br><span class="line">      &#125;</span><br><span class="line">  &#125;</span><br><span class="line">//]]&gt;</span><br><span class="line">&lt;/script&gt;</span><br></pre></td></tr></table></figure>
</li>
</ol>
<h2 id="noscript元素"><a href="#noscript元素" class="headerlink" title="noscript元素"></a>noscript元素</h2><p>noscript元素种的内容只有在下列情况下才会显示出来：</p>
<ul>
<li>浏览器不支持脚本</li>
<li>浏览器支持脚本，但脚本被禁用</li>
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